The present invention relates to a method for reducing the oxygen content and oxide inclusion content in cobalt to produce a low-oxygen cobalt sputter target having a low oxide inclusion content, and to the sputter target assemblies made therefrom.
Cathodic sputtering is widely used for depositing thin layers, or films, of materials from sputter targets onto desired substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits to form the thin layer, or film, on the receiving surface of the substrate.
Specifically, with respect to cobalt (Co) sputter targets, sputter deposition of cobalt thin films is a crucial step in forming thin and uniform cobalt silicide films. A typical CoSi2 salicide (a self-aligned silicide) process involves sputter deposition of Co thin films on silicon wafers, followed by rapid thermal processing (RTP) to form CoSi at intermediate temperatures, and sequentially to form CoSi2 at elevated temperatures. Notably, cobalt silicides have low resistivity, excellent chemical stability, inertness to nitrogen and low formation temperature and are considered an alternative to TiSi2 for use as a contact in ultra-large scale integration (ULSI).
Cobalt ingots, from which cobalt targets typically are made, contain a certain amount of oxide inclusions such as cobalt oxides, cobalt-titanium oxides, etc. These oxide inclusions, or metal defects, cause arcing during sputtering deposition and create metal particles on the substrate (i.e. silicon wafers) onto which the cobalt is sputtered thereby significantly reducing the yields of the cobalt metallization.
Today""s cobalt sputter targets typically contain high oxygen content ranging from 170 to about 400 weight parts per million (wppm) oxygen. It is known that the higher the oxygen content in a cobalt sputter target, the greater the amount of oxide inclusions. Consequently, when such high oxygen containing Co targets are used during sputtering, more arcs are formed resulting in a greater number of metal defects deposited on the substrate.
Accordingly, there remains a need in the art for a method for reducing the oxygen content and the oxide inclusion content in cobalt to produce a low-oxygen cobalt sputter target having a low oxide inclusion content, and to the sputter target assemblies made therefrom.
The present invention provides a method to reduce the oxygen content and the oxide inclusion content in cobalt to produce a low-oxygen cobalt sputter target having a low oxide inclusion content, and to the sputter target assemblies produced therefrom.
Notably, the method for reducing the oxygen content and the oxide inclusion content in cobalt are separate processes which may be combined in successive order to produce a low-oxygen cobalt sputter target having a low oxide inclusion content. The reduction in oxygen content preferably is performed prior to reducing the oxide inclusion content. Accordingly, the artisan will appreciate that one process can be performed without the other depending upon whether a reduction in oxygen or oxide inclusions is preferred in a desired cobalt sputter target. Reducing the oxygen content and oxide inclusion content in cobalt to produce low-oxygen cobalt sputter targets having low oxide inclusion contents reduces the arcing and the metal defects found with conventional high-oxygen cobalt sputter targets during sputtering.
The method for reducing the oxygen content in cobalt to produce a low-oxygen cobalt sputter target includes the steps of providing cobalt (eg. electrolytic deposit cobalt melting stock). The cobalt stock can be either low (eg. 3N5) or high-purity cobalt (eg. 4N5, 5N5, and 6N). A degassing agent, preferably carbon, more preferably carbon graphite powder, is mixed with the cobalt wherein the carbon, preferably, is present in an amount of 50-150 wppm of the mixture. The mixture is heated and degassed. The heating occurs above the melting point of the cobalt, preferably about 50 to 400 degrees F. above the melting point, to form a melted cobalt mixture wherein the carbon and initial oxygen content react to produce a second lower oxygen content. Without the addition of the carbon, the oxygen content would increase.
Finally, the melted mixture is cooled to produce a solidified cobalt having the desired lower oxygen content. The low-oxygen content in the solidified cobalt is about 1-170 ppm, preferably about 1-100 wppm, more preferably 1-about 50 and most preferably about 1-30 wppm. The solidified cobalt now is suitable for shaping into a desired sputter target or is ready for oxide inclusion reduction.
The oxide inclusion reduction method involves reducing oxide inclusions at a certain oxygen level, preferably no greater than 1000 wppm, by using certain fabrication and heat treatment steps. These steps include first providing cobalt having an initial oxide inclusion content, an initial oxygen content, and defining a first thickness. The cobalt can be either low (eg. 3N5) or high-purity cobalt (eg. 4N5, 5N5, and 6N). Accordingly, the cobalt may comprise the solidified cobalt from the oxygen reduction method.
Next, the cobalt is heated at a temperature below its melting point, preferably at about 2000 degrees F. The cobalt then is hot pressed such that the pressure reduces the first thickness to form a hot pressed cobalt defining a second thickness. The hot pressed cobalt then is heated at a temperature below the melting point thereof, preferably at about 1800 degrees F. Finally, the cobalt is hot rolled such that the rolling further reduces the second thickness to form a hot rolled cobalt defining a third thickness.
At such temperatures and at the stress provided by the press and roller, the oxide inclusions are broken into extremely small particles. These particles then dissociate to cobalt and oxygen. Finally, the dissociated oxygen is dissolved by cobalt. Accordingly, the hot rolled cobalt contains a second oxide inclusion content lower than the initial oxide inclusion content. The rolled cobalt now is suitable for shaping into a desired sputter target.
Accordingly, one object of the invention is to provide a method for reducing the oxygen content and the oxide inclusion content in cobalt to produce low-oxygen cobalt sputter targets having low oxide inclusion contents.
Another object of the invention is to reduce the arcing and metal defects associated with high-oxygen cobalt sputter targets during sputtering.
Lastly another object of the invention is to produce a sputter target assembly having a low-oxygen cobalt sputter target with a low oxide inclusion content.
The invention will be further described in the following description, the accompanying drawings and the appended claims.